This invention relates generally to printed circuit boards and the method of manufacture thereof, and more particularly to a printed circuit board structure for mounting integrated circuit chips which allows a chip having a very high density footprint to be mounted onto a circuit board using direct chip attach techniques (DCA) and providing a high density of I/O signal connections.
In the packaging of semi-conductor chips, it is required that the chips be physically mounted on to some supporting structure which not only supplies physical support for the chips, but also provides for the circuits on or in the chip, but also increasing the number of power, ground, and I/O connections which are necessary electrical connections to provide power, ground plane and, of course, I/O signals. As the technology for manufacturing I/C chips advances, it is possible to provide more and more circuits on a given chip size, thus increasing the density of the required connections. As this number of connections increases, the density of the connectors on the chip for making such connections also increases. At the present state of technology, it is well within the art to provide connectors on the chip that are on a 10-mil grid center-to-center. These chips are conventionally mounted on circuit boards along with other electrical components and indeed other chips, the circuit boards being capable of being utilized in computers or other devices which make use of the chips.
Circuit board technology does not lend itself to providing such fine line geometry as is necessary with such densely packed I/O pads, circuit board technology typically requiring at least 20 mil centers between the plated through holes (PTH) in the circuit board necessary for providing the necessary connections. While theoretically it is possible to provide the much finer plated through hole (PTH) grid on the circuit boards, nevertheless providing such fine geometry would be inordinantly expensive in those instances where it is required only for certain portions of the circuit board, i.e., where the chips are to be mounted.
Prior art solutions to this have utilized the technique of mounting chips on a chip carrier which has the fine line geometry and closely spaced pads necessary to provide connection to the chips, with the carrier providing the necessary fan-out pattern through surface and/or internal wiring and vias for subsequent connection to circuit board typically by pin-in-hole connections. This technique, while somewhat effective, nevertheless introduces a factor of requiring two levels of packaging with the attendant additional steps of processing and attendant additional cost. Thus, it is desirable and is a goal to develop techniques which allow direct chip attach (DCA) to the circuit board without the necessity of two levels of packaging.